Una Rig on IC

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Mal Smith
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Post by Mal Smith » Tue May 22, 2007 4:41 am

I'm not to clear on this, is a una rig allowed under the current IC rules? This question was put to me by a non canoe sailor who was questioning why una rigs wern't used on the IC.

Mal.
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H Virtue
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Post by H Virtue » Tue May 22, 2007 6:20 am

The IC and AC rules dont allow for it as "The area of the mainsail shall not exceed 8.5 square metres." with a total 10 square metres, this leaves 1.5 square metres to use for a jib.

The DC draft rules have no max on the main just a total of 10 square metres which is why we are seeing some UNA rigs on DC's.

Regards
H.

Phil Stevenson
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Post by Phil Stevenson » Tue May 22, 2007 11:36 am

The rule has a 0.6sq m fudge factor for "shape" plus a 75mm pocket allowance for pocket luff, so the real area of a una DC sail can be closer to 11sq m.
Mine has been chopped up so much it is proably more like 10.5 now.
I think Andy Patterson's is also smaller than allowed.
The big sail is more than enough when it is windy and also seems to go very well in the light. Still learning though.
Design perfection is achieved, not when there is nothing else to add but when there is nothing else which can be taken away.
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Mal Smith
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Post by Mal Smith » Tue May 22, 2007 11:42 pm

The person asking was my brother, Phil Smith. He is a sailmaker living in Perth and is currently working with John Illet to develop some moth sails. He was keen to try some of his new learning on an IC una rig and was trying to talk me into putting one on my boat. But I guess if I put a una rig on my IC, that would make it an overweight DC! So I'm not really considering it.

In other news, I have repaired my seat carriage since the nationals, but last time I sailed I put my feet through the mainsail during a capsize!

Mal.
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seth
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Post by seth » Sun May 27, 2007 2:31 am

Mal,

I don't think that there is any reason not to have a una rig on an IC. The only point being that you would would be limited to an 8.5(+0.6)m2 sail. If you think that the benefits of a una rig outweigh the benefits of a 1.5m2 jib...

Another option, which is not specifically governed by the rules, would be to have two masts (the way the Americans did it many years ago). If you wanted to have an unstayed mast you could perhaps have a large one at the normal position in the boat and then a small mast and sail in the foredeck.

Seth.
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Mal Smith
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Post by Mal Smith » Mon May 28, 2007 2:53 am

Seth,

I doubt that it would be worth sacrificing sail area for any efficiency gains from using a una rig except perhaps for ease of handling in strong winds. It is interesting though, to speculate on where the DC might be headed.

Aerodynamically, I perceive that the trade off between una and sloop rigs of equal area are as follows:

Upwind, the una rig will have a higher lift/drag ratio, but will not develop as much lift as a sloop rig. It is likely that the centre of effort of a sloop rig will be lower than that of a una rig. So the sloop rig will develop more power at the expense of efficiency.

Downwind, I think it is fairly clear that the Una rig will be superior in most conditions due to the more efficient spread of area.

Whether or not a una rig will pay off upwind usually depends on the lift/drag ratio of the hull and foil system, whether there are significant gains to be made from reduction of aerodynamic drag at the expense of lift. The point being that the benefits of being able to point higher into the wind must improve the VMG more than the speed gain from the added pressure. A good example of where it does pay off is the A Class cat. These boats have particularly fine low drag hulls and a high righting moment. Note also the rapid evolution of the now defunct Formula 40 multihull class down to the minimum allowable jib size. Most monohull dinghys however, where the rules allow experimentation, have retained the sloop rig, including the skiff classes. This suggests that most monohull dinghys benefit most from higher lift. This may be because the added thrust allows them to get over the typical planing hull drag hump earlier.

The IC/DC though is in an odd position somewhere between catamaran and planing monohull. So what will work best? I note that even though the minimum jib size is 1.5sqm, most IC jibs appear to be at least 2sqm in area. Does anyone have any figures to support this observation? Is this actually the most aerodynamically efficient main/jib ratio for this boat, or are there other overriding factors apart from the rules?

If the current set up is the aerodynamically best choice for the IC, will the new lighter, lower drag hull be enough to make the una rig the best choice for the DC?

Mal
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jimc
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Post by jimc » Mon May 28, 2007 12:07 pm

It seems to me that its an awful lot easier to get a two sail IC "in the groove" upwind than a reasonably equivalent single sail boat. For those with sufficient talent this is maybe not an issue. For the rest of us...

Andy P
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Post by Andy P » Mon May 28, 2007 7:48 pm

You could alternatively have:

a leading edge slat ( Handley Page flap )- approx 250mm wide x 6m long slat in front of the mast, on a 8.5m² mainsail.

or use a wing mast of 1.5m² with the max size mainsail - there is a max sail size, but the mast is not included as sail size.


or the DC rig can have one big sail

My rig is one (main)sail at about 10.7m² actual = 9.9m² measured., with a sleeve luff, and camber inducers.
The rig appears to be powerful, and also low drag - but needs testing vs other IC.

upwind it appears to be very fast against all other dinghies ( and slower cats ), with high speed and high pointing.
The sail appears very open and twisty, ( quite different to conventional IC shape with tight leech) but the boat gets fully powered up (upwind)in <10kts of breeze.

I sailed in 20kts yesterday - the top of the sail is inverted or very flat and wibbly , but I was able to sail much higher upwind than RS 400s ( normally a very high pointing boat ), and at much faster speed.


I think ( but can't tell yet) that it's faster downwind than IC sloop rig.
( but it is a bit scary downwind in 20 kts )
I notice that most IC pics downwind show a lot of spray and not much boat, so maybe that's how they go.


jib : main on sloop rig dinghies have best proportions at 33% : 67% ,( but if the boat is underpowered 25%:75% seems better )
Too big a mainsail requires more righting moment

Phil Stevenson
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Post by Phil Stevenson » Tue May 29, 2007 3:36 am

My belief is that the sloop rig will be better upwind when it is windy but that the cat rig should be better the rest of the time.

But I also think that the mast sail combination is much more critical with the cat rig. This is because as you get over powered and need to dump sheet some of the time, the cat rig still has to provide all the power, all the time, where a sloop rig main can survive even if the main is all off and on again while the jib continnues to drive the boat.

If the mast sail combo is correct small sheet movements can do the job while the sail continues to drive the boat. This applies to all rigs, its just more critical on a cat rig.

Also since not a lot of cat rigged boats are other than factory one designs, most sailmakers rarely make a cat rigged mainsail, and I believe the shape requirements are somewhat different to a main parked behind a jib's backwash. It sometimes takes a bit of negotiation to get the vertical depth distribution correct. Moth experience helps.
Design perfection is achieved, not when there is nothing else to add but when there is nothing else which can be taken away.
http://philscanoes.blogspot.com/

Kris
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Post by Kris » Tue May 29, 2007 5:17 am

Since I sail an IC I'm committed to the sloop for a while, but it is an interesting discussion.
A few careless thoughts:

Aspect ratio/efficiency: Both rigs have the same height, so aspect ratio should be roughly similar. But at least for IC jib will still have a positive effect on efficiency. If DC's end up sailing much higher and faster the sheeting angle for the jib will be very small(we are very small already). If slot is too small, better to ditch the jib.

Center of effort: Una rig has a higher CE, so will struggle more in the breeze. On the other side it will be better in light wind, more area up in the stronger breeze. Unless you heel the boat to windward the una rig will be more unbalanced downwind, and you have to correct with rudder(= drag).

Projected area downwind: Una rig winds hands down, no blanketing effect. Low aspect ratio is good for a stalled sail, definitely better than two high aspect ratio sails.

Depowering: I suspect the sloop depowers more gracefully than the una rig.

Current state of development: At this stage the sloops are more developed but that can change quickly. The pocket luff and camber inducers are very nice!

I think Phil is right that the sloop will be better upwind in a breeze and the una will be better the rest of the time. Will be very interesting to see where the break even is. I hope it doesn't lead to us sailing sloops in Aus and una in east coast US and Britain.

Paul Scott
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Post by Paul Scott » Fri Jun 01, 2007 1:34 am

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by seth</i>
<br />Mal,

Another option, which is not specifically governed by the rules, would be to have two masts (the way the Americans did it many years ago). If you wanted to have an unstayed mast you could perhaps have a large one at the normal position in the boat and then a small mast and sail in the foredeck.

Seth.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

As EMO Smith (in his Wright Bros. Lecture on high lift devices) points out, n+1 > n, and as Tom Speer points out, +1 doesn't NEED to be a jib, so maybe two masts would work. I know I said this before a long time ago, but two of the new Byte rigs would be about right as far as Sail Area, auto response, and leverage (grunt) ratios are concerned. Semi expensive, but quick and dirty. You might be able to borrow a couple (?).

Paul [:0]
"Exuberance is better than good taste" -Flaubert

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